Various graphite materials are used on a negative electrode of a lithium ion battery and a negative electrode of an electric double layer capacitor because these materials are suitable for an intercalation reaction with a lithium ion or the like. A compound to which an ion of alkali metal such as lithium or potassium, a sulfate ion or a nitrate ion is inserted between the carbon hexagonal planes (FIG. 1) by intercalation is called a graphite intercalation compound.
In lithium ion batteries and high density capacitors, electric energy is obtained by generation of a graphite intercalation compound as mentioned above, i.e., intercalation and de-intercalation of an intercalate to be inserted between the carbon hexagonal planes, and therefore, graphite materials are used as an electrode material.
In an intercalation reaction between a graphite material and an intercalant (a substance to be inserted), there is an interrelation between the degree of graphitization (crystallinity) of the graphite as a host material and the reactivity. In a donor type graphite intercalation compound where an electron is given to graphite to cause intercalation with a positive ion, reactivity is good in a host material having a high degree of graphitization. Examples of an intercalant for a donor type graphite intercalation compound are alkali metals such as lithium, sodium, potassium and cesium and alkali earth metals such as barium and strontium. (Non-patent Document 1)
These donor type graphite intercalation compounds are generally unstable in the air and react with oxygen and moisture to evacuate (de-intercalation) an intercalant from between the graphite layers. In the case of use of general graphite materials, since a size of a crystallite is as large as several microns and the materials are not complete monocrystals, a part of intercalants stay between the graphite layers to form a so-called residual compound.
In the case of using intercalation for an electrode reaction of lithium ion batteries and capacitors, it is ideal that intercalation and de-intercalation occur reversibly at high speed without forming a residual compound. In order to obtain a high charge/discharge capacity as a battery by such an ideal reaction, it is said to be necessary that a carbon material, such as a graphite material, as a host material has a high degree of graphitization, its crystallite size is fine and its form is porous thereby the area in which a reaction arises being large. (Patent Document 1)
In addition, since an intercalation reaction arises from edges of carbon hexagonal planes, it is desirable that the edges of carbon hexagonal planes are located on an outer surface of the graphite material of a host material. The reaction does not arise on a surface which is parallel to the carbon hexagonal planes. Graphite materials such as naturally produced graphite and Kish graphite precipitated when molten iron is cooled during a process for making iron, said graphite materials are precipitated from a liquid phase under high temperature and high pressure, have a large anisotropy and orientation. Carbon hexagonal planes thereof exist predominantly on the graphite particle surface and location of the edges of carbon hexagonal planes is limited only to edge area of the particle. (FIG. 7)
The carbon fibers obtained from resins such as polyacrylonitrile (PAN) or coal or petroleum pitches are drawn into a fiber and are carbonized and graphitized through the following heat treatment. Therefore, carbon fibers have a structure such that carbon hexagonal planes are oriented concentrically in parallel with each other to the fiber direction or are oriented radially to the same direction. Especially in the case of a structure having carbon hexagonal planes oriented in parallel with each other, the surface of a carbon fiber is occupied predominantly with carbon hexagonal planes. Also in the case of a carbon fiber having a radial structure, carbon hexagonal planes become surface structure for maintaining a fiber structure (the radial structure is formed as if it lies down on the surface portion of the carbon fiber), and is occupied predominantly with carbon hexagonal planes.
There are methods for producing carbon and graphite materials by vapor phase growth using hydrocarbon or hydrogen gas as a starting material and a catalyst such as metal at high temperature. Examples of carbon materials produced by vapor phase growth methods are vapor-phase-grown carbon fiber, carbon nanotube, carbon nanohorn, and fullerene. Any of them are anisotropically grown from a metal catalyst being a starting point of the reaction. Therefore, even in these vapor phase growth methods, carbon hexagonal planes exist predominantly on the outer surface of a material. (Patent Documents 2 and 3)
Example of a method for producing isotropic graphite materials is a method for kneading petroleum coke as a filler with petroleum or coal pitch as a binder and pre-molding, carbonizing and graphitizing the kneaded product using a cold isostatic pressing (CIP) equipment. These isotropic graphite materials have a porosity less than 10%, and crystallinity as a graphite is low because hard carbon for securing a strength are contained in a large amount. (Patent Document 4)
The highly oriented graphite crystal disclosed in JP 2633638 B (Patent Document 5) is a vapor-phase-grown graphite grown in glassy carbon, and is a high density graphite well grown until its bulk density reaches 2.15 or so. Therefore, a ratio of edges of carbon hexagonal planes appearing on the outer surface is relatively low and a porosity calculated from a difference between a bulk density and a true density is as low as several percent, resulting in low reaction efficiency.